Conventionally, in a multiplex transmission apparatus, a plurality of first terminal nodes, each including a first bias unit for biasing a first transmission line, out of two transmission lines (main lines), to the supply-side potential, a second bias unit for biasing a second transmission line to the ground-side potential, and a communication control unit for data communication control, and a plurality of second terminal nodes, each including a communication control unit for data communication control, are connected to one another by means of the common transmission lines. In each first terminal node, the first bias unit and the communication control unit are connected to the first transmission line by means of a first branch line, while the second bias unit and the communication control unit are connected to the second transmission line by means of a second branch line. In each second terminal node, on the other hand, the communication control unit is connected to the first and second transmission lines by means of two branch lines, individually. The first and second terminal nodes transfer data between them through these common transmission lines.
In the multiplex transmission apparatus arranged in this manner, each terminal node is connected to each transmission line by means of a joint connector and two branch lines. If one of the branch lines is cut and when data are to be transmitted and received by that terminal node which is connected to the transmission line by means of the cut branch line, the communication control unit of this terminal node is disconnected from one of the transmission lines. Therefore, the data transmission is one-way transmission by means of the uncut branch line.
If the terminal node connected to the cut branch line is the first terminal node, and when data are to be transmitted and received by means of this first terminal node, in this multiplex transmission apparatus, however, the data transmission is one-way transmission as in the aforesaid case, and bias failure is caused, resulting in a double fault. In the multiplex transmission apparatus, moreover, the transmission lines and the terminal nodes are connected to one another by means of the joint connectors and the branch lines, as mentioned before. In this multiplex transmission apparatus, therefore, there are many junctions (usually eight junctions in this connection) between the transmission lines and the terminal nodes, the first terminal node including the bias units and the second terminal node including no bias units have different impedance characteristics, and reflected waves are generated in the transmission waveform. According to the prior art example described above, the transmission lines are connected in a loop form. These problems are not, however, limited to this arrangement, and the case of bus-form connection of the transmission lines involves the same problems.